Heat dissipation from an integrated circuit (IC) die or chip during operation is typically an important issue, especially as the density of IC devices on a die continues to increase. Also, many devices now have combinations of high-power transistors and low-power transistors formed on a same die. Such high-power transistors tend to produce more heat than low-power transistors. Further, more system-on-chip configurations are being used. Thus, there are often a wide variety of IC devices on a same die. Some of the IC devices can handle and/or put out much more heat than nearby or neighboring devices on the same die. Hence, the reliability and effectiveness of heat dissipation for a packaged IC chip may greatly affect the reliability and/or performance of an IC chip during operation.
FIG. 1 shows a cross-section view of a typical die package 20 attached to a printed circuit board 22. Many IC chips 24 are housed in a package 20 having a die pad 26 (or die paddle) with an exposed side 28 (i.e., side 28 of the die pad 26 not being covered by the package plastic 30), as shown in FIG. 1 for example. Often a die pad 26 is an integral part of the lead frame structure (see e.g., lead frame 32 in FIG. 1). Generally, a die pad also may include any component that provides a thermal extension of the die pad 26 (e.g., a heat spreader or a slug). In such packaging configurations having an exposed die pad 26, the die 24 is usually adhered directly to the die pad 26 (see e.g., FIG. 1). An exposed die pad 26 is sometimes adhered to a printed circuit board (PCB) 22 to dissipate heat to the PCB 22 (see e.g., FIG. 1). Having the die 24 adhered to the die pad 26 increases the amount of heat transferred from the die 24 to the die pad 26. When a die 24 is not properly adhered to a die pad 26 or when part or all of the die 24 is not adhered to the die pad 26, the amount of heat transferred to the die pad 26 may be significantly reduced and less efficient. This is especially true when the die pad 26 is intended to be along the primary thermal path for heat dissipation from the die 24.
Thermal tests may be performed to determine whether the heat from a die 24 is being dissipated efficiently or sufficiently. In a package configuration where the die pad 26 is one of the primary heat sinks for transferring heat from a die 24 (see e.g., FIG. 1), the results of a thermal test may indicate whether a die 24 is sufficiently adhered to the die pad 26. In the past, thermal-impedance tests were performed by generating heat with the circuitry of the die 24. For example, a K-factor die or a production die that has high-power devices (e.g., motor drivers) was used to generate heat on the die 24. K-factor dies are typically used specifically for testing, and often include temperature sensing elements and resistor networks that cover most of the die surface. In production dies, the temperature of the die 24 may be derived from measuring the leakage of any parasitic diode or other silicon-based IC device because the leakage often has a linear relationship to the temperature of the die 24. Hence, certain output pins may be used as temperature sensors based upon the inherent behavior of the devices as temperature varies.
In these prior thermal testing methods (using a production die or a special testing die), the heat is generated on the die surface by internal components formed in the die 24 and the temperature is sensed (directly or indirectly) by internal components of the die 24, which are essentially at the same location (i.e., on the die 24). Using such tests, the die 24 is often driven with a relatively high power to generate enough heat for the test. Then, the amount of heat remaining on the die surface is used as an indication of the amount of heat dissipated from the die 24; presumably via the die pad 26 as a primary thermal path for some cases. However, there are many possible heat paths for dissipating heat from the die 24, other than via the die pad 26 (e.g., through leads 32, through package plastic 30). Thus, such prior testing methods may not accurately test the heat path between the die 24 and the die pad 26.